The present invention relates to a band pass filter having a plurality of helical resonators each including a quarter-wavelength winding and, more particularly, to an improved type of band pass filter having a sharper cut off characteristic in the high frequency region than in the low frequency region of the band being filtered.
The band pass filter of the above described type is employed, for example, in a television receiver set as an intermediate frequency filter. In a television receiver set, the frequency f.sub.p ' of the adjacent channel video carrier wave and the frequency f.sub.s ' of the adjacent channel audio carrier wave must be attenuated by more than 50 dB relative to the intermediate frequency f.sub.0. Since in the televised signal, the relation between the frequencies f.sub.0, f.sub.s ' and f.sub.p ' is such that f.sub.p '&lt;f.sub.0 &lt;f.sub.s ' and f.sub.0 -f.sub.p '&gt;f.sub.s '-f.sub.0, it is necessary to cut off the filter response more sharply in the high frequency region.
Conventionally, there have been proposed a number of band pass filters, and one of these is described hereinbelow with reference to FIGS. 1, 2 and 3 which show a partly cut-away perspective view, a sectional side view and an equivalent circuit diagram of the conventional band pass filter, respectively.
In FIGS. 1 and 2, the band pass filter shown includes a shield casing 1 having an elongated rectangular cubic body made of metallic plate such as aluminum which is divided into a plurality of rooms 3 by partition walls 2. Each of the partition walls 2 has a coupling opening 4 formed therein which interconnects the neighboring rooms for the purpose of aperture coupling. In each of the rooms 3, there is provided a quarter-wavelength winding 5 constituted by a coil of electrically conductive material such as copper having a few dozen turns. One end of each coil 5 is rigidly connected to a bottom wall of the casing 1 while the other end of the coil 5 is free of any of the walls. Each of the two rooms which are located at the opposite ends of the casing 1 accommodates an additional coil 6 or 7 of a type similar to that described above but having only a few turns for the purpose of loop coupling. The additional coil 6 housed in the left-hand room 3 is provided for receiving input signals from an input circuit (not shown) while the additional coil 7 housed in the righthand room 3 is provided for producing a filtered signal to an output circuit (not shown). As apparent from FIG. 2, each of the auxiliary coils 6 and 7 has one end extending outwardly from the casing 1 through a suitable insulating support 8, and the other end rigidly connected to the bottom wall of the casing 1 within the associated room 3.
The circuit diagram shown in FIG. 3 is an equivalent circuit for the band pass filter shown in FIG. 2. The parallel circuits each composed of an inductor L.sub.a and a capacitor C.sub.a correspond to the quarter-wavelength windings 5, while each of the capacitors C.sub.b corresponds to the coupling capacitance established between the neighboring quarter-wavelength windings 5 through the coupling opening 4.
So far as the propagation characteristics of any one of the parallel circuits constituted by an inductor L.sub.a and a capacitor C.sub.a is considered, it is understood that the impedance of the resonance circuit (L.sub.a and C.sub.a) drops gradually down to zero in the frequency region above the resonance frequency (hereinafter referred to as the high frequency region) and in the frequency region below the resonance frequency (hereinafter referred to as the low frequency region) since the propagation loss in the resonance circuit increases with an increase in frequency and since the resonance circuit has a definite quality factor Q. When a number of resonance circuits are coupled in series through the coupling openings 4 as described above, it is found that the cut off characteristic in the high frequency region becomes less and less sharp as the coupling capacitance of the capacitor C.sub.b increases, that is, as the distance between the neighboring quarter-wavelength windings becomes smaller or as the coupling opening 4 becomes larger.
Accordingly, as shown by the broken line curve in the graph of FIG. 9 in which the abscissa represents frequency and the ordinate represents attenuation, the conventional band pass filter of the above described type employing quarter-wavelength windings cuts off more gradually in the high frequency region than in the low frequency region.